(i) Field of the Invention
This invention relates to the production of chlorine dioxide, and more particularly to an improved integrated process for producing chlorine dioxide continuously, efficiently and rapidly with high yields and with a minimum requirement for many initial reactions.
(ii) Description of the Prior Art
Chlorine dioxide has been industrially employed as a bleaching agent by the cellulose pulp industry for over half a century. The industrial production of chlorine dioxide has grown substantially over the years. The present world production capacity approaches one-million tons annually.
The demand for chlorine dioxide is projected for significant growth in the next decade because most pulp mills are committed towards substitution of chlorine through the use of chlorine dioxide. This substitution is a result of new regulations worldwide limiting the pulp mills effluent of Total Organic Chlorides. In addition, the delignification and bleaching of pulp should be carried out without the production of chloroform, furans and dioxins.
This substitution of chlorine by chlorine dioxide represents an increased cost to pulp mills due to the higher cost of an equivalent amount of chlorine dioxide. Furthermore, the conventional chlorine dioxide plants yield, as by-products, spent acid, salt cake and sodium chlorate. This acid solution, and/or slurry (if the salt cake is crystallized) is undesirable to pulp mills because, when fed to the chemical recovery system, it can be the cause of production down time and maintenance costs associated with the boiler tubes.
One of the more traditional procedures for the production of chlorine dioxide is the reaction of hydrochloric acid and chloric acid to produce gaseous mixtures of chlorine dioxide and chlorine. In practice, these reactions were carried out by treating aqueous mixtures of water-soluble chlorates and chlorides, such as may be obtained, for example, by chlorinating lime or by the electrolysis of salt, with an excess of a strong acid, for example, hydrochloric acid or sulfuric acid. The reactions above referred to may be represented by the following equations: EQU 2NaClO.sub.3 +4HCl.fwdarw.2ClO.sub.2 +Cl.sub.2 +2NaCl+2H.sub.2 O (1) EQU 2NaClO.sub.3 +12HCl.fwdarw.6Cl.sub.2 +2NaCl+6H.sub.2 O (2)
Ordinarily, reaction (2) is predominant over reaction (1), and consequently, the yield in chlorine dioxide is correspondingly low.
To minimize reaction (2) it has been suggested to react properly proportioned mixtures of chlorates, chlorides and a strong inorganic acid in dilute solutions (containing at least 50% and preferably up to 75% of water) at temperatures below 60.degree. C. Based on reaction (1), equivalent ratios of Cl/ClO.sub.3 =2 and of H.sup.+ /ClO.sub.3 =2 should give high yields of ClO.sub.2 per mole of chlorate decomposed. In practice, however, it has been proposed in particular to use a ratio of H.sup.+ /ClO.sub.3 - in excess of 2 because reaction (2) uses some of the chlorate in producing chlorine instead of ClO.sub.2. This proposal results in the use of excessive quantities of reactants.
Since chlorine dioxide is explosive at high temperatures, the reactions described hereinabove have generally been carried out at relatively low temperatures. Furthermore, in order to reduce still further the danger of explosion, a non-reactive (inert) gas was conducted into the reaction vessel. The purpose of the gas was to reduce the concentration of chlorine dioxide in the vessel to a nonexplosive proportion.
In processes where gaseous hydrogen chloride was used or where large amounts of inert gases were used, there was a need for a rather large compressor. The need for such compressor involved increased capital and operating costs.
Of the many patents directed to the preparation of chlorine dioxide by introducing an aqueous solution of sodium chlorate and sodium chloride and an acidic agent into a reaction vessel in a continuous manner, mention be made of the following:
Canadian Patent 461,586 patented Dec. 6, 1949 by G.A. Day provided a process for the manufacture of chlorine dioxide, by supplying an aqueous solution of an inorganic chlorate to the reaction chamber, and gaseous hydrogen chloride, the hydrogen chloride being supplied to the chamber in amount insufficient to react with all the chlorate present therein. The resulting acid was reacted with the chlorate. Gaseous chlorine dioxide and chlorine were removed from the reaction chamber. Partially reacted chlorate solution from the reaction chamber was passed to an electrolytic chlorate cell to increase the chlorate content thereof. The fortified chlorate solution was returned to the reaction chamber for further reaction with hydrogen chloride. The patentee provided for the use of electrolytic chlorate liquor containing about 500 g/L sodium chlorate and about 75 g/L sodium chloride. The low amount of chloride in the electrolytic chlorate liquor did not always serve to prevent the precipitation of chloride during the reaction to form the chlorine dioxide. Such precipitation of chloride reduced the efficiency of the chlorine dioxide generator.
Canadian Patent 560,447 patented Jul. 15, 1958 by A. Germano provided a process for manufacturing of chlorine dioxide from sodium chlorate and hydrochloric acid by reacting more than two molecules of hydrochloric acid per molecule of sodium chlorate, at a temperature sufficiently low, preferably below about +10.degree. C., in such a manner as to precipitate the sodium chloride which was separated, as to desorb the chlorine dioxide and the chlorine from the solution thus obtained by the action of an inert gas and/or heat, and to re-use the residual solution for preparing the solution of chlorate for further reaction. The patentee further taught that, as a first step, a solution of sodium chlorate, preferably saturated in chlorate, and obtained by the addition of chlorate to a residue solution coming from a previous manufacture, was introduced into a reactor. The low amount of chloride in the electrolytic chlorate liquor did not always serve to prevent the precipitation of chloride during the reaction to form the chlorine dioxide. Such precipitation of chloride reduced the efficiency of the chlorine dioxide generator.
Canadian Patent 728,075 patented Feb. 5, 1966 by H.C. Scribner provided a process for the production of chlorine dioxide by reacting an aqueous solution of an alkali metal chlorate with hydrochloric acid in a reaction zone to produce chlorine dioxide and an alkali metal chloride. There was at least a portion of the solution in the reaction zone passed to an enriching zone, to provide, in the enriching zone, alkali metal chlorate in an amount in excess of the amount of chlorate required to saturate the solution with respect to the alkali metal chloride. This provided conditions favourable to the precipitation of alkali metal chloride in the enriching zone. The chlorate content of the solution was increased and the alkali metal chloride was then precipitated in the enriching zone by passing the solution through the enriching zone. The enriched solution was conducted from the enriching zone to the reaction zone for reaction with hydrochloric acid to generate chlorine dioxide. Typically, the concentration of sodium chlorate was from about 447 to about 514 g/L, the concentration of sodium chloride was from about 151 to about 202 g/L, and the concentration of hydrochloric acid was from about 17 to about 21.5 g/L.
Canadian Patent 1,076,321 patented Apr. 29, 1980 by Isao Isa et al provided a process for manufacturing chlorine dioxide by reducing a chlorate (e.g. sodium chlorate, potassium chlorate, calcium chlorate or magnesium chlorate, ) in hydrochloric acid in a concentration of from about 0.01 to about 4 moles per liter of reaction medium, in the presence of a specified complex catalyst. The concentration of the chlorate taught was from about 0.01 molar (about 10.7 g/L sodium chlorate) to about 5 molar (about 533 g/L sodium chlorate) of the reaction medium.
Canadian Patent 951,085 patented Jul. 16, 1974 by W.H. Rapson provided a process for the production of chlorine dioxide which comprised forming chlorine dioxide by reducing an alkali metal chlorate in a first acid aqueous reaction medium in a first reaction zone at a first elevated temperature below the temperature above which substantial decomposition of chlorine dioxide would occur, and simultaneously evaporating water from the first medium, thereby to form a gaseous mixture comprising steam and chlorine dioxide. The gaseous mixture was removed from the first reaction zone. Chlorine dioxide was produced by reducing an alkali metal chlorate in a second acid aqueous reaction medium in a second reaction zone physically separate from the first reaction zone at a second elevated temperature lower than the first elevated temperature. The removed gaseous mixture was passed into heat exchange relationship with the second reaction medium to provide at least part of the heat required to maintain the second elevated temperature. The chlorine dioxide formed in the first and second reaction zones was then recovered. The concentration of chlorate and chloride in the reaction medium may vary over a wide range. For example, the concentration of chlorate in the reacting solution could be in the range of about 0.005 molar (about 0.5 g/L) to about 3 molar, (about 320 g/L) and the concentration of the chloride may be in the range of about 0.001 molar (about 0.06 g/L) to about 2 molar (about 117 g/L). Consistent with such ranges, moreover, it was taught that it was preferred to utilize an approximately equimolar ratio in order to maximize the production of chlorine dioxide with equation 1 above. Ratios of chloride: chlorate generally varied between about 1:1 and about 3:1, preferably about 1.1:1 to about 1.3:1. The low amount of chloride in the electrolytic chlorate liquor did not always serve to prevent the precipitation of chloride during the reaction to form the chlorine dioxide. Such precipitation of chloride reduced the efficiency of the chlorine dioxide generator.
Canadian Patent 1,088,274 patented Oct. 28, 1980 by Alain Caillol provided an improvement in the conventional processes generating chlorine dioxide in sulfuric acid employing sulfur dioxide or methanol as reducing agent. The chlorine dioxide produced from the gaseous effluent issuing from the production reactor was separated by absorption by means of cooled water in a packed column. The gaseous effluent (tail gas) issuing from the absorption column was recycled to the production reactor, in order to dilute the chlorine dioxide produced. The improvement involved the method of reacting sulfur dioxide with chlorine in the presence of water to yield hydrochloric acid and sulfuric acid, thus, reducing chlorine to nothing, with no accumulation of chlorine in the recycled tail gases. The economy in respect of chlorate would be better still if a certain amount of chloride (not exceeding about 16% of the chlorate, preferably about 6% of the chlorate) was introduced with the chlorate solution into tile main reactor.
U.S. Pat. No. 4,137,296 patented Jan. 30, 1979 by D.N. Glew et al provided a process of producing chlorine dioxide from an alkali metal chlorate by reacting the chlorate with hydrogen chloride by maintaining, in a reaction vessel, an aqueous reaction medium containing at least about 5.5 molar (about 586 g/L) of sodium chlorate, sufficient hydrogen chloride to provide an acid normality of at least about 0.002 and sufficient sodium chloride to saturate the reaction medium. The reaction medium was boiled at a temperature of up to about 100.degree. C. A mixture including chlorine dioxide gas, chlorine gas and water vapor was withdrawn from the reaction vessel. The patentee taught the feeding of a sufficient amount of an aqueous solution containing sodium chlorate into the reaction zone to provide the desired reaction medium. A sufficient amount of hydrogen chloride into the reaction zone to provide a reaction medium acidity of from about 0.002 to about 6 normal. The reaction medium was maintained at boiling by maintaining the temperature within the range of from about 40.degree. to about 100 .degree. C. and the pressure within tile vessel within the range of from about 40 to about 500 millimetres of mercury absolute. A slurry containing a portion of the reaction medium and solid sodium chlorate particles was withdrawn from the reaction zone. Solid sodium chloride was removed from the withdrawn slurry to provide a slurry effluent. The effluent was returned to the reaction zone. Hydrogen chloride was added to the withdrawn slurry. An inert diluent gas was fed into a gas zone superimposing the reaction zone. The gaseous chlorine was withdrawn from the chlorine dioxide reaction product. The low amount of chloride in the electrolytic chlorate liquor did not always serve to prevent the precipitation of chloride during the reaction for form the chlorine dioxide. Such precipitation of chloride reduced the efficiency of the chlorine dioxide generator.
U.S. Pat. No. 4,045,542 patented Aug. 30, 1977 by W.A. Fuller provided an improvement in a process for continuously generating a mixture containing chlorine dioxide, chlorine and an alkali metal salt, wherein an alkali metal chlorate and hydrochloric acid were continuously reacted in suitable proportions to generate chlorine dioxide and chlorine with the temperature being maintained at from about 50.degree. to about 100.degree. C. The acidity of the reaction solution was maintained within the range of about 0.05 to about 2.5 normal. Chlorine dioxide and chlorine produced by the reaction was withdrawn from the reactor, and water was removed and alkali metal salt of the hydrochloric acid was crystallized in the form of an aqueous slurry containing minor amounts of chlorate, and acid values. The improvement involved continuously passing the slurry containing alkali metal salt crystals into the top of a separatory column, in a downward flow. A stream of water was countercurrently and continuously passed upwardly through the column at a rate sufficient to effect washing of the downwardly flowing crystals whereby chlorate, chloride and acid values recovered therefrom were continuously and substantially completely returned to the reaction. An aqueous slurry of substantially pure alkali metal chloride salt crystals was continuously removed from the bottom of the separatory column. The patentee further taught that the concentration of alkali metal chlorate was preferably maintained on the high side of the applicable concentration range of about 0.2 molar (about 21 g/L) to about 5 molar (about 533 g/L). This was especially true during operation in the region from approximately 75.degree. C. to 90.degree. C. and at pressures in the region of 400 millimetres mercury absolute. The low amount of chloride in the electrolytic chlorate liquor did not always serve to prevent the precipitation of chloride during the reaction for form the chlorine dioxide. Such precipitation of chloride reduced the efficiency of the chlorine dioxide generator.
Improvements were also provided in the inventions described in the above identified patents by providing a continuous process for the production of chlorine dioxide involving the recycling and reuse of by-products and incompletely-reacted reagents. Among such patents are:
Canadian Patent 782,574 patented Apr. 9, 1968 by G.O. Westerlund, which provided an improved continuous recyclic process and apparatus for the production of chlorine dioxide. That patented process involved the first steps of effecting electrolysis of an aqueous solution of a metal chloride to form an aqueous solution of a metal chlorate and gaseous hydrogen. The gaseous hydrogen was reacted with gaseous chlorine to form gaseous hydrogen chloride. The aqueous solution of metal chlorate was reacted with the gaseous hydrogen chloride to form an aqueous solution of metal chloride, which was recycled to the first step and an aqueous solution of chloric acid. The aqueous solution of chloric acid was reacted with the gaseous hydrogen chloride to form chlorine dioxide, which was recovered, water and gaseous chlorine, which was recycled to the second step.
That patented process for the manufacture of chlorine dioxide which was based on the following main reactions: (M=metal ion) ##STR1##
The action of hydrogen chloride on the metal chlorate solution produced chlorine dioxide and chlorine according to reactions (3) and (4). By controlling the acidity and by utilizing an excess of the metal chlorate, reactions (3) and (4) can be caused to yield ClO.sub.2 :Cl.sub.2 in a ratio of 2:1. Chlorine was consumed in reaction (2) for production of hydrogen chloride. Thus the system theoretically yielded chlorine dioxide only, free of chlorine to the extent of the efficiency of chlorine dioxide gas separator. Since the system was closed, after the initial charge of metallic chloride, no addition of salt was required. Furthermore, the electrolysis in reaction (1) produced three times the required amount of hydrogen. The process of that invention was thus based on a system which required water, chlorine and electric current for the production of chlorine dioxide.
The system of that invention was a useful improvement over the previous systems, being a closed system with essentially no major effluent liquor other than the output of chlorine dioxide solution. Therefore, losses of reagents were minimized and the manufacturing cost of chlorine dioxide was determined by the cost of power and chlorine. The system in that invention was self-regenerating in regard to chemicals except for chlorine and water. At 100% yield, one mole of chlorine was required for the production of two moles of chlorine dioxide. The only other raw material was electrical energy, the two moles of water per mole of ClO.sub.2 being insignificant.
Other improvements were provided by the following patents:
Canadian Patent 808,556 patented March 18, 1969 by G.O. Westerlund provided an improvement in the chlorine dioxide preparation process of Canadian patent 782,574 by the steps of reacting the inorganic chlorate with an aqueous hydrochloric acid solution whereby chlorine dioxide gas and chlorine gas were formed. Chlorine gas was separated from chlorine dioxide. A portion of the chlorine gas was reacted with hydrogen gas to form hydrogen chloride. Hydrogen chloride was dissolved in water to form an aqueous solution of hydrochloric acid. The aqueous hydrochloric acid produced was then used as one of the reactants.
Canadian Patent 809,556 patented Mar. 25, 1969 by G.O. Westerlund provided another improvement in the chlorine dioxide preparation process of Canadian Patent 782,574 by the step of reacting the inorganic chlorate with an aqueous solution of hydrochloric acid. The gaseous products of reaction comprising chlorine and chlorine dioxide were swept from a gas zone atop the liquor zone with a diluent gas admitted to the reaction zone near the interface between the gas zone and the liquor zone.
Canadian Patent 828,061 patented Nov. 25, 1969 by G.O. Westerlund provided still another improvement in the chlorine dioxide production process of Canadian Patent 782,574 by the step of diluting hydrogen chloride gas with chlorine or a mixture of chlorine, carbon dioxide and water vapor. The aqueous solution of chlorate was reacted with the diluted hydrogen chloride gas. A final reaction product comprising about 10% or less chlorine dioxide gas diluted with about 90% or more chlorine gas, or about 10% or less chlorine dioxide gas diluted with about 90% or more of a mixture of chlorine gas, carbon dioxide gas and water vapour was then recovered.
Canadian Patent No. 922,661 patented Mar. 13, 1973 by G.O. Westerlund provided an integrated process for the continuous, high yield production of chlorine dioxide. The process included the step of effecting an electrolysis reaction in an electrolysis zone to form a gaseous product comprising predominantly hydrogen, as well as water, oxygen and chlorine, and a liquid product comprising an aqueous solution of a chlorate, a chloride and a dichromate. Both liquid product and gaseous effluent were withdrawn from the electrolysis zone. The withdrawn electrolysis products was passed to a reaction zone where the liquid produced, comprising an aqueous solution of a chlorate and a chloride, was converted to a product comprising an optimum proportion of chlorate therein. Gaseous product comprising hydrogen and well as steam, oxygen, and chlorine substantially-free of the liquid product was withdrawn from the reaction zone. The withdrawn liquid product was then passed to a two-stage chlorine dioxide generation zone, each stage being maintained within different temperature ranges where the chlorate liquor was reacted with aqueous hydrochloric acid reducing agent in the presence of a sweeping current of air and a portion of the gaseous product. Liquid product was withdrawn from the second stage of the chlorine dioxide generation zone and such withdrawn product was passed to an evaporation zone where a controlled portion of the water therein was evaporated to provide hot water for heat use, and from where weak chlorate liquor was withdrawn for recycling to the electrolysis zone. Gaseous effluent containing liquid product was withdrawn from the second stage of the chlorine dioxide generation zone, and the contained liquid was separated therefrom. Such liquid was conducted to the evaporation zone. The gaseous effluent was passed countercurrently in contact with an aqueous solvent in a chlorine dioxide adsorption zone. Liquid effluent comprising an aqueous acidic solution of chlorine dioxide was withdrawn from the chlorine absorption zone and such effluent was stored in a chlorine dioxide solution storage zone. Gaseous effluent was withdrawn from the chlorine dioxide absorption zone.
U.S. Pat. No. 3,607,027 patented Sep. 21, 1971 by G.O. Westerlund provided an improved process for preparing chlorine dioxide wherein the bulk of the reactants were internally produced. The reactants included an aqueous solution of an inorganic chlorate and an aqueous hydrochloric acid. The aqueous solution of the inorganic chlorate was produced by electrolyzing an aqueous solution of an inorganic chloride. The aqueous hydrochloric acid was produced from hydrogen gas, which was a by-product of the electrolysis reaction by which the inorganic chloride was converted to the inorganic chlorate, and chlorine gas. In reacting the aqueous inorganic chlorate with the aqueous hydrochloric acid, both gaseous chlorine dioxide and gaseous chlorine were formed. The gaseous chlorine was separated from the gaseous chlorine dioxide. A portion of the so-separated gaseous chlorine was reacted with the hydrogen to form gaseous hydrogen chloride. The gaseous hydrogen chloride was used as a reactant, either by dissolving it in a stoichiometrically insufficient quantity of water, thereby to provide an aqueous solution of hydrochloric acid and free hydrogen chloride gas, or by dissolving it in water containing absorbed chlorine gas.
U.S. Pat. No. 3,929,974 patented Dec. 30, 1975 by J.D. Winfield provided a process for the production of chlorine dioxide by continuously feeding an aqueous solution of an alkali metal chlorate and hydrochloric acid to a reaction zone to maintain an aqueous reaction medium in the reaction zone containing an alkali metal chlorate and hydrochloric acid and having an acidity of about 0.05 to about 1N. Chlorine dioxide and chlorine were continuously formed by reaction between the alkali metal chlorate and the hydrochloric acid in the reaction medium. The reaction zone was continuously maintained under a reduced pressure. The reaction medium was continuously maintained at its boiling point to evaporate water from the reaction medium continuously to form a gaseous phase in the reaction zone consisting of a mixture of chlorine dioxide, chlorine and water vapor, and to deposit alkali metal chloride in the reaction zone. The gaseous phase mixture was continuously conducted out of the reaction zone. Chlorine dioxide was continuously recovered from the mixture. Deposited alkali metal chloride was removed from the reaction zone. An aqueous solution was formed from the removed alkali metal chloride. That aqueous solution was continuously electrolyzed to convert the alkali metal chloride at least partially to alkali metal chlorate and to generate hydrogen gas. There was at least the alkali metal chlorate content of the alkali metal chlorate-containing solution continuously fed to the reaction zone. There was at least part of the hydrogen gas continuously reacted with chlorine gas to generate hydrogen chloride. Hydrochloric acid was continuously formed from the hydrogen chloride. There was at least part of that continuously fed hydrochloric acid to the reaction zone. The patentee further taught that the concentration of sodium chlorate could be about 250 g/L and that the concentration of sodium chloride was about 150 g/L. The low amount of chlorate in the electrolytic chlorate liquor did not allow for the optimum concentration to optimize the production of chlorine dioxide. High concentrations of chlorate tended to increase the efficiency of the production of chlorine dioxide.